Method of determining the angular position of a permanent magnet rotor of a polyphase electric motor

ABSTRACT

This method consists in supplying at least a first alternating signal, at a determined frequency, to one end of a stator coil and recuperating a measurement signal at a second end of said coil, said measurement signal being provided to electronic processing means, which are arranged to extract data relating to a periodic variation of the effective inductance of the stator coil. This variation is a function of the angular position of the rotor. By alternately carrying out such a measurement on the stator coils, three periodic curves can be extracted (SPC A , SPC B , SPC C ) defining a zigzag curve ( 70 ) allowing the angular position of the rotor to be determined.

BACKGROUND OF THE INVENTION

The present invention concerns a method for determining the angularposition of the permanent magnet rotor of a multiphase electric motorincluding at least two stator coils. The exact determination of theangular position of the rotor of an electric motor is desired, or evennecessary for several applications, in particular in the field ofrobotics and for devices with rotating tools such as those used indental care and micro-surgery.

There is known an arrangement of three Hall sensors, which are offset by120° and which provide substantially three sinusoidal curves offset by120°, in order to determine the angular position of a permanent magnetrotor. It is also possible to arrange a position encoder on the shaft ofthe rotor to obtain a certain precision in determining the rotor'sangular position. These arrangements require the addition of sensorswhich make the device more bulky, which is a major drawback for devicesof small dimensions integrating micro-motors.

There is also known a generator providing induced voltage signals whoseamplitude is proportional to the rotational speed of the rotor. Thus,for a three-phase permanent magnet rotor, it is possible to stop thepower supply of the motor periodically for short intervals in order toextract induced voltage data allowing the position and/or speed of therotor to be determined. This latter method for determining the angularposition of a rotor has a major drawback given that the data can only beobtained when the rotor rotates at a certain speed, for exampleapproximately 1,000 revolutions per minute. Thus, at low speed or forrotations of a relatively small angle, no data can be deduced on thebasis of the induced voltage in the motor coils.

SUMMARY OF THE INVENTION

The object of the present invention is to overcome the drawbacks of theaforementioned prior art by avoiding an angular position detectiondevice which is bulky and by providing a method for detecting theangular position of a rotor whatever its rotational speed and theangular path travelled.

This object is achieved by a method for determining the angular positionof the permanent magnet rotor of a multiphase electric motor includingstator coils which are respectively associated with the phases of thismotor, each stator coil having a first end and a second end, wherein:

an alternating signal, having a determined frequency, is supplied to thefirst ends of at least two of said stator coils.

measurement signals in response to said alternating signal arerecuperated, via conductor means connected to the second ends of said atleast two stator coils, and supplied to electronic processing means,these measurement signals having a periodic amplitude modulation due toa periodic variation in the effective inductance of the correspondingcoil as a function of the rotor's angular position, two measurementsignals, coming from two different phases of the motor, thus havingenvelopes of corresponding amplitude modulations which arephase-shifted.

an extraction from each recuperated measurement signal of said envelopeof amplitude modulation is made by said electronic processing means.

at least two of said envelopes which are phase-shifted are use fordetermining said rotor's angular position which is a function of saidperiodic amplitude modulation of each measurement signal.

The object of the present invention is also achieved by a method fordetermining the angular position of a permanent magnet rotor of amultiphase electric motor including stator coils which are respectivelyassociated with the phases of this motor, each stator coil having afirst end and a second end, wherein:

a first alternating signal and a second alternating signal, both havinga determined frequency, are simultaneously and respectively supplied tothe first ends of two of said stator coils, said second alternatingsignal being phase-shifted relative to said first altemating signal,

a measurement signal, generated by mixing signals received at the secondends of said two stator coils in response to said first alternatingsignal and said second alternating sianal, is recuperated, via conductormeans connected to these second ends, and supplied to electronicprocessing means,

said measurement signal is processed by said processing means in orderto obtain a periodic resulting signal which corresponds to a periodicphase variation of this measurement signal relative to said firstalternating signal due to the periodic variation in the effectiveinductance of said two stator coils as a function of the rotor's angularposition,

the rotor's angular position is determined from said periodic resultingsignal.

Within the developments having led to the present invention, theinventor has observed that this measuring signal includes data relatingto a periodic variation in the effective inductance of the coil in whichthe alternating signal is supplied as a function of the angular positionof the permanent magnet rotor, in particular of bipolar permanent magnetrotor.

The invention also concerns the electronic system for implementing themethod according to the invention.

Via appropriate processing of the measuring signal, the method accordingto the invention allows the rotor's angular position to be determinedwith a high level of precision whatever its rotational speed and evenwhen it is not moving. As a result of this method, it is possible tocontrol the motor so as to make relatively small paths with a high levelof precision.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be better understood with the aid of thefollowing description of implementations of the method according to theinvention and means for implementing the same, made with reference tothe annexed drawings, which are given by way of non-limiting example,and in which:

FIGS. 1 and 2 show the inductance variation in a stator coil via afrequency variation in an LC oscillator associated with the statorcoils;

FIGS. 3 and 4 show respectively two embodiments of the rotor used withinthe scope of the present invention;

FIG. 5 partially shows a first embodiment of a system for detecting arotor's angular position according to the invention;

FIGS. 6 to 9 show a second embodiment of a detection system according tothe invention, used to explain a preferred implementation of the rotorposition detection method according to the invention;

FIG. 10 shows schematically a circuit for correcting the measurementcurves used in the method according to the invention;

FIGS. 11 and 12 show a method according to the invention for selecting ameasurement value among three measuring signals obtained;

FIG. 13 shows a first variant for determining the rotor's angularposition as a function of the measurement value selected; and

FIG. 14 shows schematically the means for implementing a second variantfor determining the rotor's angular position as a function of themeasurement value selected.

DETAILED DESCRIPTION OF THE INVENTION

The electronic circuit shown in FIG. 1 forms an LC oscillator in whichinductance is defined by coils 2 a, 2 b and 2 c of an electric motorwhose rotor 3 includes a bipolar permanent magnet 4. The circuit of FIG.1 enables an oscillation to be maintained varying, in particular,between 2.8 MHz and 3 MHz as a function of the rotor's position, as isshown schematically in FIG. 2. It will be observed that this variationis substantially independent of the rotor's rotational speed or thevalue of the supply current supplied to the motor. A periodic variationis thus observed in the effective inductance of the coils of theelectric motor essentially as a function of the rotor's position. Thoseskilled in the art can design an electronic circuit for measuring thefrequency of an oscillator of the type shown in FIG. 1 in order todeduce therefrom data relating to the rotor's angular position.International Patent Application No. WO 00/04631 by the same inventoruses the measurement of the frequency variation of two oscillatorsassociated with the stator coils. For more ample explanations concerningthe electronic circuit of FIG. 1, reference will also be made toEuropean Patent Application No. EP 99125017.6 whose priority is claimed.In this European Patent Application, rotor embodiments are alsodisclosed like those shown here in FIGS. 3 and 4 for increasing theperiodic variation in the effective inductance of the coils as afunction of the rotor's angular position.

FIGS. 3 and 4 show two alternative embodiments of a rotor 3A and 3Bfitted to an electric motor associated with an angular positiondetection system according to the invention. The Applicant has observedthat the arrangement of a metal, in particular a non-magnetic metal,partially on the surface of the rotor's permanent magnet, increases theperiodic variation in the effective inductance of the stator coils.Rotor 3A has two metal strips 5 offset by 180°, only one of the twostrips being visible in FIG. 3. Rotor 3B has a metal cylinder 6surrounding permanent magnet 4 and in which are provided two rows ofholes 7 arranged axially and also offset by 180°. In this variant shownby way of non-limiting example, the N-S magnetic axis is aligned on theradial direction of holes 7. For more ample details, reference will alsobe made to the aforementioned European Patent Application.

With reference to FIG. 5, a first implementation of the method accordingto the invention and the system for implementing said method will bedescribed hereinafter.

A frequency generator 10 provides through a band-pass filter 12 analternating signal having a determined frequency, which is selectivelysupplied to a first end of one of the three stator coils 2 a, 2 b and 2c. Next, a measurement signal is recuperated via another of these coils,which is connected to the second end of the coil into which thealternating signal is supplied. This measurement signal is provided toelectronic processing means including an amplifier 18 at the input ofwhich there is provided a resistor R. Given that the input impedance ofamplifier 18 is very high, resistor R of lower resistance value is usedto lower the impedance at the input of said amplifier to prevent theappearance of disturbance in the measurement signal. It will be notedhowever, that the resistance value of resistor R is much higher than theresistance value of a coil so that the voltage signal forming themeasurement signal at the input of amplifier 18 substantiallycorresponds to the voltage of the signal having the frequency of saidalternating signal at the middle point 20 of the three stator coils.

In order to obtain the cleanest possible measurement signal, the latterpasses through a bandpass filter 22 at the output of which analternating signal can be observed, whose envelope has a modulationwhich is a function of the rotor's angular position. In order to extractthe data relating to this angular position, the measured signal passes,in a conventional manner, through a rectifier 24 in order to obtain anelectric signal corresponding to the aforementioned envelope. Given thatthe alternating signal is successively sent, in a cyclical manner, overthe three phases A, B and C of the electric motor, three measurementsignals are obtained which define three curves during rotation of therotor, these curves having a substantially sinusoidal shape and beingoffset or phase shifted by 120° as appears in window 26. However, it hasbeen observed that, given that the three stator coils are not alwaysidentical, the three measurement curves obtained have a different offsetand amplitude. Thus, means are provided for correcting the offset andamplitude of each curve. These means include a summing operationalamplifier 28 to one input of which the measured signal is provided,whereas a specific offset signal for each of the three phases A, B and Cis supplied to its second input via a digital-analogue converter 30. Thespecific offsets of the three phases on which measurements are made arestored in a register 32. Finally, the measured signal is provided to amultiplier digital-analogue converter 34 whose multiplicationcoefficient CM is specifically selected for each of the three phases ina register 36. The activation of analogue switches 16 and selection ofthe offset and coefficient of multiplier CM are performed simultaneouslyin a cyclical manner using an electronic control circuit 38.

Thus, using the three measurement curves defining three measurementvalues for each angular position of the rotor, it is possible todetermine this angular position precisely. If the curves aresubstantially sinusoidal, this determination can occur digitally withoutany great difficulty. If this is not the case, methods for efficientlydetermining the angular position of the rotor will be describedhereinafter with reference to the preferred embodiment of the invention.

A study of the sensitivity of the system described hereinbefore forimplementing the angular position detection method according to theinvention has shown that it has certain drawbacks. First, the electroniccircuits for obtaining three measurement curves having the same offsetand same amplitude are relatively complex. Next, the system environment,in particular the electric cables associated with the motor, modifiesfrom one case to another the measured amplitude modulation and theoffset of the measurement curves. Thus, a second preferredimplementation of the invention and embodiment of the electronic systemassociated therewith, wherein a phase modulation is used, will bedescribed hereinafter.

According to the preferred implementation of the invention, which willbe described hereinafter with reference to FIGS. 6 to 10, a first signalSF_(A) is supplied to a first phase of the motor, i.e. to a first inputof a stator coil and a second alternating signal SF_(B) of the samefrequency FO is simultaneously supplied to a second phase, i.e. to afirst end of a second coil. Signal SF_(B) is offset relative to signalSF_(A) by means of a phase-shift circuit 42. The phase shift betweenthese two alternating signals is predefined. These signals SF_(A) andSF_(B) are supplied to the first ends of two coils through two bandpasscircuits 12 and 13 and amplifiers 14 and 15 as in the first embodimentdescribed hereinbefore.

Via the third coil associated with the third phase of the motor, ameasurement signal SM_(c) is obtained which is supplied to electronicprocessing means, in particular a converter, in order to extract thedata due to the periodic inductance variation as a function of therotor's angular position. Measurement signal SM_(c) is formed by mixingthe signals received at point 20 of the electric connection between thethree stator coils. In response to the first and second alternatingsignals SF_(A) and SF_(B), a voltage measurement signal is thusobtained, the frequency of which corresponds to the frequency FO ofthese signals SF_(A) and SF_(B). Signal SM_(c) is amplified by amplifier18 and filtered at frequency FO by filterinE means 22 before beingsupplied to a trigger 46 which then supplies a digitalised signal SDC,which is an image of measurement signal SM_(c).

Provided signal SFA and the envelope of measurement signals SM_(C)amplified for a half revolution of bipolar magnet 4 of rotor 3 are shownin window 48. It will thus be observed that the phase of signal SM_(C)relative to alternating signal SF_(A) varies as a function of therotor's angular position α between two extreme values defining a maximumphase shift of Δθ. By digitalising the substantially sinusoidal curvesof signal SM_(C), trigger 46 allows the phase shift variation betweensupplied signal SF_(A) and measurement signal SM_(C), to be measured, asis shown schematically in FIG. 7. Alternating signal SF_(A) has a timeperiod TP whose leading edge defines an initial reference time T0.Digitalised measurement signal SD_(C) has a leading edge 50 which occursafter a variable time interval TV from initial time T0.

It is the variation in interval TV that contains data relating to theeffective inductance variation. In order to exploit this data mostefficiently, two measurements are provided, as shown in FIG. 8A. First,an RC circuit 54 is provided, for supplying a voltage proportional tothe ratio TV/TP. In order to increase the variation in voltage signalURC_(c), a three-state gate 56 is introduced to enclose the positionvariation of leading edge 50 within a time window T whose value is alittle higher than the maximum TV variation. Thus, the variation insignal URC_(c) substantially varies between a low value and a valueclose to URCmax. The time window occurs after a predefined delay timeT_(ret). Three-state gate 56 is activated by a control signal FTsupplied through a “one shoot” circuit 58 by a delay circuit 60.Circuits 58 and 60 are known to those skilled in the art. FIG. 8B showsthe various signals occurring in the processing of the measurementsignal to measure its phase variation. The electronic processing meansthus define a phase discriminator and supply a voltage signal whosevariation corresponds to the phase variation of measurement signalSM_(c). As shown in FIG. 9, signal URC_(c) is also supplied to anoperational amplifier 62 so as to reduce the impedance of themeasurement signal at the output of the amplifier. Thus signal UMP_(c)variable as a function of the angular position of the rotor and having aperiod corresponding to half a revolution of this rotor is obtained.Thus the measured and processed signal allows a curve having a periodicvariation and a shape like a sinusoidal curve to be obtained.

It will be noted that the phase shift introduced by circuit 42 betweenthe two signals supplied for measurement has a value which is preferablybetween 140 and 160°. This range of values corresponds to a compromisebetween an optimum phase variation in the measured signal and itsvoltage.

In order to measure the rotor's angular position efficiently and inparticular its rotational direction, it is necessary to have at leasttwo measurement curves offset in relation to each other. In order to dothis, one proceeds in a similar manner to the first embodimentpreviously described in FIG. 5, by supplying the first and secondalternating signals cyclically and successively to two of the threestator coils so as to obtain three measurement signals whose evolution,as a function of the rotor's position, respectively defines threemeasurement curves offset by 120° with respect to each other. In thepresent case, measurement signals are successively recuperated on eachof the three phases A, B and C by supplying the two offset alternatingsignals to the two other phases. Those skilled in the art know how tomake circuits for recuperating, cyclically and successively inrelatively short time intervals, the three measurement signals at thesecond ends of the three stator coils defining the three phases of theelectric motor. In order to do this, it is possible to use, inparticular, operational amplifiers with a high impedance three-stateoutput, for example of the CLC430 type of the National company. A set ofswitches of this type can be controlled by an “FPGA” switching circuitof the XC4008 type of the Xilinx company.

Thus, for each angular position of the rotor, it is possible to obtainthree measurement signals. By successive measurements, the rotation ofthe rotor defines three periodic measurement curves generated by thephase modulation or phase shift variation of these measurement signalsin the three coils of the electric motor as a function of the angularposition of the permanent magnet of the rotor.

Given the slight lack of symmetry between the three phases, using adynamic corrector, correcting the measurement signals so that themeasurement curves generated have the same offset and the same amplitude(see FIG. 10) is proposed. In order to do this, signal UMP supplied byeach of the three phases is supplied to a converter 66 which is alsocontrolled by switching the aforementioned FPGA circuit. Dynamiccorrection is performed using a microprocessor which supplies correctedmeasurement values SPC for each of the three phases. Three curvesSPC_(A), SPC_(B) and SPC_(C), offset by 120°, are thus obtained.

Within the scope of the present invention, a precise and efficientmethod for determining the rotor's position has been conceived. Thismethod will be explained hereinafter with reference to FIGS. 11 to 14.It was observed that in the peak regions of the measurement curves,given the relatively small slope, the accuracy of measurement decreases.In order to eliminate these regions, according to the invention, for agiven position of the rotor, the measurement signal whose value islocated between the values of the other two measurement signals, or isequal to the value of one of these two other measurement signals, isselected from the three measurement signals. This means that, for agiven angular position a single measurement value is selected, on asegment of the zigzag curve 70 defined by the bold line in FIG. 11.

Over a half-revolution of the rotor, the angular position is definedunivocally, using an algorithm to select segments of curve 70, given inFIG. 12 in the first three lines. Indeed, for a measurement value givenby one of curve segments 70 belonging to a determined phase, this valuecan correspond over a half-revolution to only two angular positions. Forthese two angular positions, the two values taken by one of the othertwo measurement signals are respectively lower than and higher than saidvalue of curve 70. The last four conditions given by the algorithmdefined in FIG. 12, to determine the rank of the segment of zigzag curve70 over a complete revolution of the rotor, take account of theevolution of the measurement signals over time. In other words, thevalue REG put in register 72 at time t-1 is stored in a memory and theevolution of value REG at a time t allowing two successivehalf-revolutions to be distinguished depending on the logic state of bitz3 is observed. Thus, by giving a different rank to each segment ofcurve 70 over a rotational revolution of the rotor, it is then possibleto determine, for a measurement value selected on one of the segments ofcurve 70 whose rank is determined, the angular position of the rotorunivocally over a complete revolution, as is shown by curve 74 in FIG.11.

In order to determine the value of the angular position as a function ofthe measurement value selected on zigzag curve 70, two variants areproposed. The first variant defines an analytical method schematicallyshown in FIG. 13. For a given electric motor or a particular type ofmotor, a reference zigzag curve is defined corresponding to curve 70.This reference curve is given using an analytical formula.

In the variant proposed in FIG. 13, reference zigzag curve 74 is formedof a succession of linear segments defining an approximation of thecorresponding real curve. Thus, in the reference curve, the segments ofcurve 70 are associated with linear segments whose parameters aredefined for example using a linear regression on the real measurementcurves obtained for the motor of type of motor concerned. For eachlinear segment SGM_(N) of rank N, extreme points (PIP_(N); PIA_(N)) and(PIP_(N+1); PIA_(N+1)) and slope TGS_(N) are predefined and introducedinto the electronic processing means. For a segment whose rank N isdetermined, the angular position α_(N)(t) as a function of measurementvalue A_(N)(t) is given by the following analytical formula:${\alpha_{N}(t)} = {\frac{{A_{N}(t)} - {PIA}_{N}}{{TGS}_{N}} + {PIP}_{N}}$

The second variant for determining the angular position consists in acomparative method. A plurality of reference values, defining areference curve over at least a half-revolution of the rotor, isintroduced into a correspondence table 76. A predefined rotor angularposition value, also stored in table 76, corresponds to each referencevalue. Once the segment of curve 70 has been determined, the angularposition is obtained by determining which is the closest reference valueto the selected measurement value.

In order to define the aforementioned reference values, a positionencoder 78 can be used and the curves UMP obtained for each phase bydigitalising them by means of a convener 76 as shown schematically inFIG. 14.

Finally, although the implementations of the method according to theinvention have been described with reference to motors with stator coilsarranged in a star, this method also applies to a triangular connectionof the stator coils.

What is claimed is:
 1. A method for determining the angular position ofthe permanent magnet rotor of a multiphase electric motor includingstator coils which are respectively associated with the phases of thismotor, each stator coil having a first end and a second end, comprisingthe steps of: supplying an alternating signal, having a determinedfrequency, to the first ends of at least two of said stator coils,recuperating measurement signals, in response to said alternatingsignal, via conductors connected to the second ends of said at least twostator coils, and supplying the measurement signals to electronicprocessing means, the measurement signals having a periodic amplitudemodulation due to a periodic variation in the effective inductance ofthe corresponding coil as a function of the rotor's angular position,two measurement signals, coming from two different phases of the motor,thus having envelopes of corresponding amplitude modulations which arephase-shifted, extracting, by said electronic processing means, fromeach recuperated measurement signal a corresponding one of saidenvelopes of amplitude modulation, and using at least two of saidphase-shifted envelopes for determining said rotor's angular position asa function of said periodic amplitude modulation of each measurementsignal.
 2. The method according to claim 1, further comprising the stepof supplying said alternating signal successively and cyclically to thefirst ends of said at least two stator coils.
 3. The method according toclaim 1, wherein said motor has three phases, and further comprising thestep of supplying said alternating signal to the first ends of threestator coils respectively associated with the three phases.
 4. Themethod according to claim 2, wherein said motor has three phases, andfurther comprising the step of supplying said alternating signal to thefirst ends of three stator coils respectively associated with the threephases.
 5. A method for determining the angular position of a permanentmagnet rotor of a multiphase electric motor including stator coils whichare respectively associated with the phases of this motor, each statorcoil having a first end and a second end, comprising the steps of:simultaneously and respectively supplying a first alternating signal anda second alternating signal, both having a determined frequency, to thefirst ends of two of said stator coils, said second alternating signalbeing phase-shifted relative to said first alternating signal,recuperating a measurement signal, generated by mixing signals receivedat the second ends of said two stator coils in response to said firstalternating signal and said second alternating signal, via conductorsconnected to these second ends, and supplying the measurement signal, toelectronic processing means, and processing said measurement signal bysaid processing means to obtain a periodic resulting signal whichcorresponds to a periodic phase variation of the measurement signal,relative to said first alternating signal, due to the periodic variationin the effective inductance of said two stator coils as a function ofthe rotor's angular position, and determining the rotor's angularposition from said periodic resulting signal.
 6. The method according toclaim 5, wherein said electric motor is a three-phase motor and includesthree coils respectively associated with three phases, furthercomprising the step of supplying said simultaneous first and secondalternating signals cyclically and successively to the three coils so asto obtain three measurement signals whose evolution, as a function ofthe position of the rotor, respectively defines three measurement curvessimilar to sinusoidal curves offset by 120° with respect to each other.7. The method according to claim 6, further comprising the step ofcausing said electronic processing means to correct the amplitude andoffset of each measurement curve so as to obtain three correctedmeasurement curves having the same amplitude and centered on the samemean value, said three corrected measurement curves defining correctedmeasurement values for the three measurement signals.
 8. The methodaccording to claim 7, further comprising the step of causing saidelectronic processing means: to select, for a given position of therotor, the measurement signal from said three measurement signals whosecorrected value is situated between the corrected values of the twoother measurement signals or is equal to one of said two other correctedvalues, said corrected values of said selected signal defining aselected measurement value; to determine the rank of a curve segmentfrom the plurality of curve segments forming a zigzag curvecorresponding to all measurement values capable of being selected over aperiod corresponding to a complete rotation of the rotor; and todetermine, for the selected measurement value and the segment ofdetermined rank, univocally the value of said angular position.
 9. Themethod according to claim 8, further comprising the steps of providing,in a memory of said electronic processing means, predefined parametersof an analytical formula defining a reference zigzag curve, anddetermining said angular position by a calculation algorithm byintroducing said selected measurement value into said analyticalformula.
 10. The method according to claim 9, further comprising thestep of forming said reference zigzag curve by a succession of linearsegments.
 11. The method according to claim 8, further comprising thesteps of including in said electronic processing means a correspondencetable into which a plurality of predefined reference values areintroduced, introducing a predefined angular position value for eachreference value into said correspondence table, and obtaining saidangular position by determining which is the closest reference value tosaid selected measurement value.
 12. The method according to claim 5,further comprising the step of arranging said electronic processingmeans to follow the evolution of the rotation of the rotor so as toprovide the angular position of the rotor univocally over a range of360°.
 13. The method according to claim 8, further comprising the stepof arranging said electronic processing means to follow the evolution ofthe rotation of the rotor so as to provide the angular position of therotor univocally over a range of 360°.
 14. The method according to claim6, further comprising the step of supplying said simultaneous first andsecond alternating signals successively and cyclically in relativelyshort time intervals.
 15. The method according to claim 6, furthercomprising the step of providing a set of switches, controlled by aswitching circuit of the “FPGA” type, to carry out the step of supplyingsaid first and second alternating signals.